1. Field of the Invention
The present invention relates to an automatic transmission and more particularly to an automatic transmission having a plurality of transmission gear trains between input and output shafts.
2. Discussion of Prior Art
Generally, a manual transmission, in which the gear is manually shifted, has an input shaft directly connected to an engine and including a plurality of drive gears and has an output shaft including a plurality of driven gears paired with the corresponding drive gears and connected to drive wheels. That is, there are provided a plurality of transmission gear trains between the input shaft and the output shaft. In this manual transmission, when the gear is changed, after a clutch is disengaged, changeover mechanisms such as synchromesh mechanism are manually operated to change over paired transmission gear trains (hereinafter, referred to as just gear trains) and then the clutch is engaged. This sequence of manual operations accomplishes a gear shift of a vehicle.
The manual transmission can be converted into an automatic transmission by performing gearshiftings and clutch operations by means of shift actuators hydraulically operated. This type of transmission having a plurality of gear trains is called an Automated Manual Transmission (hereinafter referred to as xe2x80x9cAMTxe2x80x9d). The transmission has advantages in having a small number of components and an excellent transmission efficiency of power, compared to a conventional torque converter type automatic transmission primarily constituted by planetary gears and the like.
For example, Japanese Patent Application Laid-open No. Toku-Kai 2000-65199 discloses this AMT type automatic transmission having a plurality of gear trains provided between an input shaft and an output shaft, a changeover mechanism for changeover gear trains, that is, selecting a gear train bearing power transmission from a plurality of gear trains and a bypass clutch for mediating power transmission from the input shaft to the output shaft in a synchronizing manner with the changeover operation of gear trains.
Further, Japanese Patent Application Laid-open Toku-Kai-Hei 6-316230 proposes an automatic transmission in which an input shaft is connected with an output shaft through gear trains by selectively engaging a plurality of hydraulic multiple disc clutches provided in respective gear trains.
However, these automatic transmissions need a reverse drive gear provided on the input shaft and a reverse driven gear meshing with the reverse drive gear through an idler gear and provided on the output shaft. That is, a dedicated reverse gear train is needed, this resulting in an increase of the number of components and an enlarged space for accommodating the reverse gear train.
It is an object of the present invention to provide an AMT type automatic transmission capable of reducing the number of gears of gear trains and the longitudinal size of the automatic transmission.
To achieve the object, an automatic transmission for a vehicle having a plurality of gear trains provided between an input shaft and an output shaft, comprises a hollow shaft coaxially including the input shaft, a particular forward drive gear rotatably mounted on the hollow shaft, a forward driven gear integrally mounted on the output shaft and constantly meshing with the forward drive gear, a reverse drive gear rotatably mounted on the input shaft adjacently to the forward drive gear, a friction clutch provided between the input shaft and the hollow shaft for selectively transmitting power from the input shaft to the hollow shaft, a changeover hub integrally mounted on the hollow shaft between the particular forward drive gear and the reverse drive gear, an first idler gear constantly meshing with the reverse drive gear, a second idler gear integrally formed with the first idler gear and constantly meshing with the forward driven gear and a changeover member provided between the particular forward gear and the reverse drive gear for engaging the changeover hub either with the particular forward drive gear or the reverse drive gear.